The synthesis of various multi-arm radial or star polymers have become of growing practical and theoretical interest to a variety of industries. Such star polymers are seen as useful as, inter alia, surfactants, lubricants, rheology modifiers, and viscosity modifiers. In fact, star polymers are now considered by many to be state-of-the-art viscosity modifiers and oil additives, although the potential of some of these star polymers for these applications is still being evaluated and tested.
One well-known representative of this class of materials currently being used as an oil additive is commercially available from the Shell Oil Co. (Houston, Tex.) under the tradename Shelvis. This oil additive is a multi-arm star molecule consisting of many hydrogenated polyisoprene arms emanating from an ill-defined core of crosslinked polydivinylbenzene (PDVB). By the term "ill-defined" it is meant that the core of the star polymer, e.g., PDVB, is an uncontrolled, crosslinked, gel-like structure having unsaturation sites in the core. In comparison, "well-defined" cores are built of readily characterizable, soluble molecules which are precursors to the core. As a result, the structure of the resultant star polymers having well-defined cores can be controlled.
Also, it is believed that the resultant star polymers having well-defined cores may impart better shear stability than star polymers using ill-defined cores. That is, the presence of unsaturation sites (i.e., double bonds) in the ill-defined cores (PDVB) provides for the possibility that the resultant star polymers will be less shear stable and more sensitive to oxidative reactions than the star polymers having well-defined cores. Thus, in engine oil where shear stability is of critical importance, the possibility exists that during high temperature use and heavy shear in the engine, the ill-defined cores will degrade.
Similarly, the polyisoprene arms in the Shelvis product may contain some unsaturation which is also undesirable. Like the core, the presence of double bonds in the arms may cause them to decompose as well during high temperature and heavy shear conditions within the engine.
Recently, there has been a growing interest in star polymers consisting of multiple polyisobutylene (PIB) arms. For example, Kennedy et al. U.S. Pat. No. 5,395,885 describes the synthesis of star polymers having multiple PIB arms and PDVB cores using cationic synthesis techniques. Because the structure of polybutylene is readily characterized and contains no unsaturation, these PIB-based stars are suspected to be useful for a variety of applications such as motor oil additives and viscosity index improvers. However, their potential is still being evaluated and tested, and in motor oil additives where shear stability is of critical importance, the possibility remains that, because of the use of ill-defined, crosslinked aromatic cores such as PDVB, the PIB--PDVB stars currently being tested may not be highly desirable for such use.
On the other hand, studies have demonstrated that silicone oils apparently have superior shear stability properties as compared to hydrocarbon oils. For example, Fitzsimmons et al., in Trans. ASME, 68, 361 (1946), have shown that the viscosity of a silicone oil decreased less than 2 percent after 105,000 cycles, whereas the viscosity of a hydrocarbon oil dropped by more than 50 percent after only 18,000 cycles under certain operating conditions in an aircraft gear pump. Thus, it is seen as highly desirable to provide a star polymer having multiple well-defined polyisobutylene arms and a well-defined, silicone-based core which polymer would be shear-stable.
However, heretofore, polyisobutylenes have been thought to be incompatible with silicone compounds. That is, the polyisobutylenes have a tendency to segregate from siloxane compounds when mixed. Thus, linking polyisobutylene arms to siloxane cores to form multi-arm polyisobutylenes has heretofore never been accomplished.
Beyond the advantages described hereinabove, it is believed that resultant star polymers having polyisobutylene arms and siloxane cores will be more acid-base stable than other siloxane-containing compounds. For example, it is well known that Si--O groups are easily hydrolyzable in the presence of strong acids. However, it is believed that the polyisobutylene arms of the subject polymer protect the Si--O groups from hydrolytic attack, thereby aiding in the acid-base stability of the polymer.
Furthermore, because it would be a star polymer, the siloxane-containing polyisobutylene composition would have a lower viscosity and higher molecular weights than other siloxane-containing polymers. Such properties are seen as being highly desirable for a variety of applications.